Download 4.4 Notes “How is a rock`s absolute age determined?” Objective

4.4 Notes
“How is a rock’s absolute age determined?”
Objective: Define absolute age in rocks and fossils. Describe ways
used to measure it.
Absolute age is the true age
of a rock or fossil. Absolute
age tells scientists the
number of years ago a rock
layer formed.
What is meant by absolute
age?
Radioactive elements give
off particles and energy
which causes the elements
to decay. As the elements decay, new elements form that may or
may not be radioactive.
The rate radioactive decay happens can be measured. Radioactive
elements are like natural clocks.
What is radioactive decay?
Half-life is the time it takes for one-half of the mass of a
radioactive element to decay.
Example: A form of uranium called U-238 decays into lead. If you
begin with 6 kg of U-238, 3 kg will decay into lead after 4.5 billion
years. After another 4.5 billion years, only 1.5 kg of U-238 would
be left. By comparing the amount of the radioactive element in a
rock to its decay element, scientists can find the absolute age of a
rock.
If a rock has equal amounts of lead and U-238 in it, how old would
the rock be?
Carbon-14 is a radioactive form of
carbon used to date the remains of
living things. It is used to find the
absolute ages of wood, bones, and
“young” fossils. If a fossil is more than
50,000 years old, the process cannot be
used because almost all of the carbon14 has decayed into nitrogen-14.
What is carbon-14 used for?